In recent years, it has become commonplace to include suspension systems on bicycles, both for the front wheel and the rear wheel. Many different types of suspension systems exist, and most employ a spring and/or a damper of one type or another. Typically, front wheel suspension systems and rear wheel suspension systems are different one from the other in terms of specific design and construction. Rear wheel bicycle suspensions do not need to accommodate steering, but must be able to absorb a substantial amount of force, typically much more than the front wheel, as there is more weight distributed to the rear wheel, especially when accelerating.
In mountain biking and in downhill bicycle racing, it is extremely important that the rear wheel suspension be able to absorb a considerable amount of force in the event of the rear wheel impacting the ground, in order to protect the rider and to stabilize the kinetics of the bicycle, thus helping to maintain control of the bicycle. Indeed, with many prior art rear wheel suspension systems, it is common for the suspension system to "bottom out" due to the landing impact experienced by the rear wheel. In other words, the range of motion of the rear wheel suspension system is completely taken up, and any force that is not absorbed up to that point is then directly transmitted to the frame of the bicycle and, presumably, ultimately to the rider, which is extremely uncomfortable and could cause injury. Further, such extreme forces could easily change the kinetics of the moving bicycle, thus reducing the control that the rider has over the bicycle, which is unacceptable in mountain biking or downhill bicycle racing.
All known prior art rear wheel suspension systems, as discussed below, employ a single spring and/or damper arrangement to absorb shock. It is well known that a single spring and/or damper arrangement cannot absorb both low impact forces, such as those encountered while riding along slightly rough terrain, and also high impact forces, such as those encountered in rough terrain or when landing after being airborne. The use of a non-linear spring, or equivalent geometric system, helps accommodate this problem, but with unacceptable results. Further, while the simple compression or extension of a non-linear spring is adjustable, the non-linearity of such a spring is not adjustable. In other words, a suspension system employing a single spring, even a non-linear spring, has only a simple adjustment and, accordingly, can not be adjusted via "fine tuning" of the suspension system to properly absorb both low impact forces and high impact forces. Such fine tuning is extremely important in mountain biking and downhill bicycle racing, as it is dependent on the terrain and on the weight of the rider, among other factors.